This invention relates generally to detecting water leaks in a steam generator used in a nuclear reactor containing liquid sodium coolant, and more particularly to a method and an apparatus for continuously measuring concentrations of hydrogen in the cover gas, i.e., argon gas, used in a nuclear reactor.
In the steam generator used in a nuclear reactor containing liquid sodium coolant, water is heated by high temperature liquid sodium to generate steam. When a water leak occurs in a pipe wall which separates liquid sodium from water, a violent reaction takes place between water and liquid sodium and this can result in failure or damage of the steam generator. Hydrogen gas is produced in the course of the reaction, and is then transferred to the argon gas phase with which the space above the free surface of the liquid sodium is filled. Therefore, it is possible to detect water leaks in the steam generator system by continuously monitoring the hydrogen concentration in the argon cover gas.
In order to minimize damage to the steam generator system and to maintain a safe operation of the system, rapid and sensitive detection of hydrogen in the argon cover gas is important.
Prior methods of detecting the hydrogen concentration in argon gas include gas chromatograhy and a method using a metal membrane. In the gas chromatograph method, as shown in FIG. 1, a sample 1 of gas is auotmatically and intermittently picked up from the argon cover gas by a sampler 2, and the sample gas is introduced together with a carrier gas 3 into a column 4 to separate hydrogen from the argon cover gas. Then, the separated and eluted hydrogen and carrier gas are respectively subjected to a measurement of their thermal conductivities by two thermal conductivity cells 5, and the concentration of hydrogen in the argon can be read from the difference between these two thermal conductivity values, and can be recorded by a recorder 7. The column 4 and thermal conductivity cells 5 are contained in an oven 6.
To make the measurement by this system rapid, the system has been designed to make the retention time of the hydrogen as short as possible, on the assumption that no impurity gases other than hydrogen exist in the sample gas, so as to measure hydrogen concentrations at time intervals of about three minutes. This system, therefore, has the following disadvantages:
1. If there are impurity gases, other than hydrogen in the sample gas, mainly gases such as oxygen and nitrogen, they are simultaneously eluted with the hydrogen since they cannot be completely separated from hydrogen. This leads to a positive error in the numerical indication of hydrogen concentration.
2. In order to completely separate impurity gases from hydrogen, it is necessary to measure hydrogen concentrations at time intervals of more than five minutes. Therefore, this system is useless for quickly detecting the occurrence of gases produced as a result of the reaction between water and liquid sodium, i.e., occurrence of water leaks.
3. Measurement of hydrogen concentrations according to this system is made intermittently, not continuously.
4. The apparatus used in this system is so large and complicated that the places where it can be installed are limited and the cost of the apparatus is high.
The prior method using a metal membrane utilizes the characteristic that hydrogen can selectively permeate and diffuse through a membrane of nickel or palladium. One apparatus which is often used in this method is shown in FIG. 2. The membrane tube 11 is positioned within a chamber 13 in an electric furnace 12 and the tube 11 and chamber 13 are maintained at a temperature of 450.degree. to 500.degree.C. On the other hand, the insides of both the membrane tube 11 and a hydrogen gas detector 14 (e.g., an ionization gauge) are kept at a high vacuum by means of a vacuum pump 15 and diffusion pump 16. An orifice 17 is interposed between the exhaust side of the membrane tube 11 and the detector 14, in order to maintain the inside of the vacuum system at a constant vacuum. When argon gas containing hydrogen is introduced into the chamber 13, only hydrogen permeates through the membrane and enters through the inside of the tube 11 into the detector 14, where the hydrogen concentration is continuously detected as a change of degree of vacuum.
This prior method, however, has the following disadvantages:
1. In order to increase the permeation of hydrogen through the metal membrane, the thickness of the metal membrane is reduced as much as possible, for example until it has a thickness of 50 to 100 .mu.. Therefore, the metal membrane is easily cracked, deformed and burst under thermal stress.
2. It is assumed in this method that only hydrogen can selectively permeate through the metal membrane and that the changes of the degree of vacuum in the hydrogen detector are produced only by hydrogen contained in the sample gas. However, when microscopic cracks have been produced in the metal membrane, a very small amount of argon can also permeate through the cracks and enter the hydrogen detector. In addition, the sensitivity of the ionization gauge is increased as the mass number of the gas which is in contact with the gauge becomes larger. For these reasons, there is a possibility that noticeable errors can occur in the indication of the hydrogen concentration.
3. If the sample gas contains hydrocarbon gases and/or sodium vapors, they attach to the surface of the metal membrane and cause a decrease of the permeation of hydrogen through the membrane. Furthermore, they cause the surface of the membrane to burst due to the cementation thereof.
4. It is impossible to foresee the deterioration of the metal membrane and the production of the microscopic cracks therein.